Lighting Question

[Bug-Rex]

I am looking to add about 4-6 more driving lights to the front of my car. I was informed that if I do this then my alternator will struggle to provide enough power to keep my car and all its electrical components running together.

Is this true? If so what can I do to prevent this from happening? I don't really want to fork out $500 for a high output alternator, so what other options are there?

The lights will be a pair of Hella 700FF's and a pair of Hella 500FF's

Thanks much!

[carl]

4 more lights should be ok. The most I ever ran was 6 total (4 700ff, 2 500ff) - at idle things would dim (with the STI alternator which is a bit more than a wrx). If you convert them to HIDs you can run quite a few (I would not recommend converting 700 or 500s to HID - I tried it with poor results. 4000's are amazing with HIDs, that's the set-up I swapped to for a while). Currently I run 4 PIAA HIDs with no dimming at idle

[Bug-Rex]

Thanks Carl. So you think I will be OK with the stock alt with 4 extra lights?

I won't be idling much when they are turned on either so I think that will help.

[williaty]

As carl said, on a 2.0L WRX, you're going to be very, very marginal with 4 55w bulbs added. You're going to wear out a lot of things sooner and have problems with inrush current when it's cold.

However, a 160A alternator from Maniac Electric is around $300. That's the way to go (with the required correct wiring).

[carl]

As carl said, on a 2.0L WRX, you're going to be very, very marginal with 4 55w bulbs added. You're going to wear out a lot of things sooner and have problems with inrush current when it's cold.

However, a 160A alternator from Maniac Electric is around $300. That's the way to go (with the required correct wiring).
I thought I had found a 180A somewhere - it wasn't too expensive either. I think I lost that bookmark on my last laptop though

[Bug-Rex]

What about a 140A from Maniac? Would it suffice?

[williaty]

The 160A is built out of the OEM case for the 2.5L engines and is more efficient, especially at idle. Go with the 160.

[Bug-Rex]

Thanks for the help! I'll definitely look into it.

Would I be hurting anything if I just ran two more 500FF's or 700FF's for now on the stock alternator?

[williaty]

You'd probably be ok. You have less headroom on the 2.0L alternator than I do on the 2.5L alternator, but I think you'd probably be ok.

[Bug-Rex]

Cool. Thanks for the info. DirtyImpreza to the rescue again!

[indytruckboy]

Would a 90 amp STI alternator suffice? Or a 110 amp FXT alternator?

[williaty]

With my 90A stock alternator, I could run the stock lows+highs, 2x55w driving lights and 2x55w fog lights. However, when the AC compressor cycled on, everything would dim as the electromagnetic clutch in the AC locked up.

With my 160A Maniac Electric alternator, I run stock lights, 2x55w fogs, 2x100w drivings, and nothing makes them dim at all.

[Carl S]

Amps = Watts / Volts
So a 55w light at 14 volts draws just under 4 amps.
Multiply times 4 lights and you get 15.7 amps.
At 12v the draw becomes 18.3 amps for 4 55w lights.

[williaty]

Amps = Watts / Volts
So a 55w light at 14 volts draws just under 4 amps.
Multiply times 4 lights and you get 15.7 amps.
At 12v the draw becomes 18.3 amps for 4 55w lights.

Doesn't work that way. The load of the lamp is constant. As supply voltage drops, so does current flow. 55w@13.8VDC=3.46Ohms w/ 3.99A draw. 3.46Ohms@12v=3.47A.

So current drops from 3.99A to 3.47A as voltage sags from 13.8 to 12 VDC.

[Carl S]

Ak, you're right. Thats what I get for trying to think over spring break!

I think what I was meaning to get at was that if the lights are rated 55w at 12v then they'll draw 4.58A at 12v and their resistance is 2.618 Ohms. At 14v the power output would be 74.86w with a draw of 5.35A.

So yeah, same thing you're saying, but the amperage required depends on what voltage the 55w is rated at, which I believe is usually 12v for car lighting.

[williaty]

Even though we call it 12V, all car stuff is actually rated at 13.8VDC.